The present invention relates to an integrated front projection display system. In particular, the present invention relates to a low-profile integrated front projection system that coordinates specialized projection optics and an integral screen optimized to work in conjunction with the optics to create the best viewing performance and produce the necessary keystone correction.
Electronic or video display systems are devices capable of presenting video or electronically generated images. Whether for use in home-entertainment, advertising, videoconferencing, computing, data-conferencing or group presentations, the demand exists for an appropriate video display device.
Image quality remains a very important factor in choosing a video display device. However, as the need increases for display devices offering a larger picture, factors such as cost and device size and weight are becoming vital considerations. Larger display systems are preferable for group or interactive presentations. The size of the display system cabinet has proven an important factor, particularly for home or office use, where space to place a large housing or cabinet may not be available. Weight of the display system also is an important consideration, especially for portable or wall-mounted presentations.
Currently, the most common video display device is the typical CRT monitor, usually recognized as a television set. CRT devices are relatively inexpensive for applications requiring small to medium size images (image size traditionally is measured along the diagonal dimension of a rectangular screen). However, as image size increases, the massive proportions and weight of large CRT monitors become cumbersome and severely restrict the use and placement of the monitors. Also, screen curvature issues appear as the screen size increases. Finally, large CRT monitors consume a substantial amount of electrical power and produce electromagnetic radiation.
One alternative to conventional CRT monitors is rear projection television. Rear projection television generally comprises a projection mechanism or engine contained within a large housing for projection upon the rear of a screen. Back-projection screens are designed so that the projection mechanism and the viewer are on opposite sides of the screen. The screen has light transmitting properties to direct the transmitted image to the viewer.
By their very nature, rear projection systems require space behind the screen to accommodate the projection volume needed for expansion of the image beam. As background and ambient reflected light may seriously degrade a rear projected image, a housing or cabinet generally encloses the projection volume. The housing may contain a mirror or mirrors so as to fold the optical path and reduce the depth of the housing. The need for "behind-the-screen" space precludes the placing of a rear projection display on a wall.
A new category of video presentation systems includes so-called thin Plasma displays. Much attention has been given to the ability of plasma displays to provide a relatively thin (about 75-100 mm) cabinet, which may be placed on a wall as a picture display in an integrated compact package. However, at the present time, plasma displays are costly and suffer from the disadvantages of low intensity (approx. 200-400 cd/m.sup.2 range) and difficulty in making repairs. Plasma display panels are heavy (.about.80-100 lbs., .about.36-45 kg.), and walls on which they are placed may require structural strengthening.
A traditional type of video presentation device that has not received the same degree of attention for newer applications is front-projection systems. A front-projection system is one that has the projection mechanism and the viewer are on the same side of the screen. Front projection systems present many different optical and arrangement challenges not present in rear projection systems, as the image is reflected back to the audience, rather than transmitted. An example of front projection systems is the use of portable front projectors and a front projection screen, for use in meeting room settings or in locations such as an airplane cabin.
One of the advantages of front projectors is the size of the projection engine. Electronic front projectors traditionally have been designed for the smallest possible "footprint", a term used to describe the area occupied on a table or bench, by the projector. Portable front projectors have been devised having a weight of about 10-20 lbs. (.about.4.5-9 kg.).
Nevertheless, front projection systems have traditionally not been considered attractive for newer interactive applications because of factors such as blocking of the image by the projector or the presenter, poor image brightness, image distortion and setup difficulties.
Traditional electronic front projectors typically require a room that may afford the projection volume necessary for image expansion without any physical obstructions. Although images may be projected upon a large clear flat surface, such as a wall, better image quality is achieved by the use of a separate screen. FIGS. 1 and 2 illustrate a traditional front projection system. A projector 10 is placed on a table or other elevated surface to project an image upon a screen or projection surface 20. Those familiar with the use of electronic projectors will appreciate that tilting the projector below the normal axis of the screen produces a shape distortion of the image, known as a keystone effect. Most new electronic projectors offer a limited degree of keystone correction. However, as may be appreciated in FIG. 2, the placement of the projector may still interfere with the line of sight of the audience.
Of greater significance is the fact that to achieve a suitable image size, and also due to focus limitations, the projector 10 requires a certain "projection zone" in front of the screen 20. Table A lists the published specifications for some common electronic projectors currently in the market.
TABLE A Smallest Shortest Maximum Projector Lens Focal Imager Screen Throw Throw Keystone Type Length Diagonal Diagonal Distance Ratio Correction CTX Opto * 163 mm 1.0 m 1.1 m 1.1 20.degree. EzPro 580 Transmissive offset/ LCD optical InFocus * 18 mm 1.3 m 1.5 m 1.2 18.degree. LP425 Reflective offset DMD Chisholm 43-58.5 23 mm 0.55 m 1.2 m 2.2 15.degree. Dakota X800 mm Reflective electronic LCD Epson 55-72 mm 33.5 mm 0.58 m 1.1 m 1.9 * Powerlite Transmissive 7300 LCD Proxima 45-59 mm 23 mm 0.5 m 1.0 m 2.0 12.degree. Impression Tranmissive offset A2 LCD 3M 167 mm 163 mm 1.0 m 1.2 m 1.2 16.degree. MP8620 Transmissive offset/ LCD optical *Not given in published specifications
Throw distance is defined as the distance from the projection lens to the projection screen. Throw ratio usually is defined as the ratio of throw distance to screen diagonal. The shortest throw distance available for the listed projectors is one meter. To achieve a larger image, between 40 to 60 inches (.about.1 to 1.5 meters), most projectors must be positioned even farther away, at least 8 to 12 feet (approximately 2.5 to 3.7 meters) away from the wall or screen.
The existence of this "projection zone" in front of the screen prevents the viewer from interacting closely with the projected image. If the presenter, for example, wishes to approach the image, the presenter will block the projection and cast a shadow on the screen.
Traditional integrated projectors require optical adjustment, such as focusing every time the projector is repositioned, as well as mechanical adjustment, such as raising of front support feet. Electronic connections, such as those to a laptop computer, generally are made directly to the projector, thus necessitating that the projector be readily accessible to the presenter or that the presenter runs the necessary wiring in advance.
Another problem with front projectors is the interference by ambient light. In a traditional front projector a significant portion of the projected light is scattered and is not reflected back to the audience. The loss of the light results in diminished image brightness. Accordingly, a highly reflective screen is desirable. However, the more reflective the screen, the larger the possible degradation of the projected image by ambient light sources. The present solution, when viewing high quality projection systems such as 35 mm photographic color slide presentation systems, is to attempt to extinguish all ambient lights. In some very critical viewing situations, an attempt has been made even to control the re-reflection of light originating from the projector itself.
Some screen designers have attempted to address the ambient light problem with "mono-directional reflection" screens, that is, a projection screen attempts to absorb light not originating from the projector, while maximizing the reflection of incident light originating from the direction of the projector. Nevertheless, since portable projectors are, in fact, portable and are used at various throw distances and projection angles, it has proven very difficult to optimize a screen for all possible projector positions and optical characteristics.
An alternative is to design a dedicated projection facility. Such a design necessitates a dedicated conference room, in which the projector and screen position, as well as the projector's optical characteristics, are rigorously controlled and calibrated. Structural elements may be used to suspend the selected projector from the ceiling. Once calibrated, such system would be permanently stationed. Such a facility may suffer from high costs and lack of portability.
Another issue that prevents optimal performance by front projectors is the keystone effect. If projectors are placed off-center from the screen, keystoning will occur. Keystoning is a particular image distortion where the projection of a rectangular or square image results in a screen image that resembles a keystone, that is a quadrilateral having parallel upper and lower sides, but said sides being of different lengths.
Methods for the reduction of keystoning again are dependent upon the position of the projector with respect to the screen. Keystone correction may be achieved by optical and by electronic methods. For large keystone correction in LCD imagers, optical methods are presently preferable, as electronic methods may suffer from pixelation distortion, as pixels become misaligned. Presently, to the applicants' knowledge, the available optical keystone correction in commercially available portable electronic front projectors is between 10.degree. to 20.degree..
The need remains for a large screen video presentation system that offers efficient space utilization, lower weight and attractive pricing. Such a system should preferably yield bright, high-quality images in room light conditions.